One woofer wants ~38L tuned ~35Hz to get a decent response down low. This is much bigger than I'd like to use, so what about going isobaric?

Cabinet volume would be ~18L (plus a bit). I'd want to tune around 40Hz. Normally, this would require a very long port to get enough area to avoid chuffing. However, the amplifier I'll be using will be an Amp6-basic from 41Hz. It'll be fed from a 12v battery of some kind (probably one of those deep-cycle SLAs). That'll give me a few watts, sure, but not much compared to "normal" subwoofers.

Even feeding the full 50w into an isobaric pair, I can use a 20mmx100mm port 165mm long and the maximum vent speed maxes out at ~23m/s. This seems acceptable to me. The port is also fairly small, so won't eat into cabinet volume much.

The coils from each woofer would be paralleled, then its one 4ohm woofer to each amplifier channel.

I'd do a LP and HP filter built into the woofer, so signal can be run into that, then out to the BMRs, stopping them doing LF.

I have some 9mm MDF available to me. Normally I'd want to use thicker, but I think, if braced correctly, it'll be rigid enough while keeping weight down.

Does any of this seem really bonkers/stupid? I might've missed something obvious.

With 50w total, WinISD predicts ~97dB @40Hz, and ~100dB for the rest of the range, at 1m.
The Equal Loudness Contours suggest this'll be quite audible.

The 97dB@1m is absolute peak value. It corresponds to 80 phon on the ELC scale. In other words it the correct output if the rest of the music peaks at 80dB.

However, since we have to talk about average values and not peaks in music. The average will be 9-10dB lower than the average value. Being generous let's just say the average is 88dB. That's equal to 66 phon on the ELC, or correspons to saying that the maximum volume you can listen to your music at where the sub can actually be heard is 66dB, or slightly louder than normal conversation level.

Remember that unlike indoors where you have the room lift at lower frequencies to compensate for our hearings inability to pick up lower frequencies, outdoors we have no such lift, so the ELC has to be observed vigilantly.

Now the above was all at 40hz but it illustrates the futility in making small sized subs with very limited output levels to go particularly deep for outdoors use.

Instead I'd go for speakers that are much more efficient and aim for decent performance to about 80hz, and then let psychoacoustic bass effect fill in the rest. Much more productive and gives far better result.

Sub-40Hz content is recorded at roughly -12dB. It seems like a fair assumption to guess that there's some attenuation at 40Hz, though I've no idea how much... Probably best not to rely on that, and just be pleased if it does work in my favour.

I'd like to stick with these woofers as I have them already. If I used them, the only cost to me would be a couple of TL072s and a battery.
I do have another driver that's not doing anything: a Faital 6FE200. Its a 6" PA midbass. Quite efficient, but needs 14L to hit 80Hz. I save quite a lot of power using this, but lose quite a lot of bass.

How's this for an idea:

using the 8"ers, build an isobaric cabinet, but have two ports, and a rubber plug.

- Tune one port to ~65Hz, which gives a ~5dB rise in the 70-80Hz range, better for outdoors (the added efficiency there means I only gain a couple of dB sensitivity by going for the 6"). The ~105dB output capability at 80Hz puts the peaks around 95 phon, so the average volume will be more like 85. This seems more usable: 66dB isn't enough.
- Tune one port ~45Hz, giving a fairly flat response, better for indoors, up against a wall.

Block one or the other depending on situation.

I know that more efficient speakers are the way to go, but spending as little as possible is the priority here.

My suggestion would be to use my standard formula for optimum portable BR tuning.

That would give a cabinet of 22.63L internal volume not including volume taken up by the drivers and port(s). Port tuned to 49.06hz. Use 2nd order LP at 121hz

Peak would be at 94hz. Usable range from 59 to 128hz. Peak output would be much more usable 112dB. And average would correspond to a quite reasonable 96dB average listening level.

In general, since you're not going to be anywhere near output levels where the bass can be physically felt except the kick bass in the 90-100hz region. You're better off forgetting to reproduce it, and focus on it being good at what it does and fill in with psychoacoustic bass instead.

My suggestion would be to use my standard formula for optimum portable BR tuning.

That would give a cabinet of 22.63L internal volume not including volume taken up by the drivers and port(s). Port tuned to 49.06hz. Use 2nd order LP at 121hz

Peak would be at 94hz. Usable range from 59 to 128hz. Peak output would be much more usable 112dB. And average would correspond to a quite reasonable 96dB average listening level.

In general, since you're not going to be anywhere near output levels where the bass can be physically felt except the kick bass in the 90-100hz region. You're better off forgetting to reproduce it, and focus on it being good at what it does and fill in with psychoacoustic bass instead.

It gives a far better result. I cannot stress this enough.

I rather like those numbers. This is with the two drivers mounted conventionally, right?